This invention relates to a wear-resistant hard facing composition applied to wear surfaces on teeth on bits for drilling oil wells or the like.
Bits for drilling oil wells and the like commonly have a steel body which is connected at the bottom of a drill string. Steel cutter cones are mounted on the body for rotation and engagement with the bottom of a hole being drilled to crush, gouge, and scrape rock for drilling the well. One important type of rock bit referred to as a milled tooth bit has roughly triangular teeth protruding from the surface of the cone for engaging the rock. The principal faces of such a milled tooth that engage the rock are usually dressed with a layer of hard facing material to resist wear. The specific tooth geometry forms no part of this invention.
Conventional hard facing usually comprises particles of tungsten carbide bonded to the steel teeth by a metal alloy. In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. Most hard facing on rock bits employs steel as the matrix, although other alloys may also be used.
It is quite common in referring to the material in the hard facing merely as "carbide" without characterizing it as tungsten carbide. The metal carbide principally used in hard facing is tungsten carbide. Small amounts of tantalum carbide and titanium carbide may be present, although considered to be deleterious. It will be understood that as used herein, reference merely to "carbide" means tungsten carbide.
A typical technique for applying hard facing to the teeth on a rock bit is by oxyacetylene or atomic hydrogen welding. A welding "rod" or stick is formed of a tube of mild steel sheet enclosing a filler which is primarily carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. The hard facing is applied by melting an end of the rod on the face of the tooth. The steel tube melts to weld to the steel tooth and provide the matrix for the carbide particles in the tube. The deoxidizer alloys with the mild steel of the tube.
Three types of tungsten carbide have been employed for hard facing. Possibly the most common is crushed cast carbide. Tungsten forms two carbides, WC and W.sub.2 C and there can be an essentially continuous range of compositions therebetween. Cast carbide is typically a eutectic mixture of the WC and W.sub.2 C compounds, and as such is substoichiometric, that is, it has less carbon than the more desirable WC form of tungsten carbide. Cast carbide is frozen from the molten state and comminuted to the desired particle size.
Another type of tungsten carbide is so-called macro-crystalline tungsten carbide. This material is essentially stoichiometric WC in the form of single crystals. Most of the macrocrystalline tungsten carbide is in the form of single crystals. When larger particle sizes are examined, it is found that some bicrystals of WC are formed. Macrocrystalline WC is desirable for its toughness and stability.
The third type of tungsten carbide used in hard facing comprises cemented tungsten carbide, sometimes referred to as sintered tungsten carbide. Cemented tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Cemented tungsten carbide is made by mixing tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and "sintering" the composite at temperatures near the melting point of cobalt. The resulting dense cemented carbide can then be comminuted to form particles of cemented tungsten carbide for use in hard facing.
Although mild steel sheet is used when forming the tubes, the steel in the hard facing as applied to a rock bit is a hard, wear resistant, alloy steel. This occurs by reason of the oxidizers such as silicon and manganese mixed in the filler in the tube and dissolution of tungsten, carbon, and possibly cobalt, from the tungsten carbide during welding. There may also be some mixing with alloy steel from the teeth on the cone.
It is important to provide as much wear resistance as possible on the teeth of a rock bit cutter cone. The effective life of the cone is enhanced as wear resistance is increased. It is desirable to keep the teeth protruding as far as possible from the body of the cone since the rate of penetration of the bit into the rock formation is enhanced by longer teeth (however, unlimited length is infeasible since teeth may break if too long for a given rock formation). As wear occurs on the teeth, they get shorter and the drill bit may be replaced when the rate of penetration decreases to an unacceptable level. It is desirable to minimize wear so that the footage drilled by each bit is maximized. This not only decreases direct cost, but also decreases the frequency of having to "round trip" a drill string to replace a worn bit with a new one.
Thus, advances in wear resistance of hard facing are desirable to enhance the footage a drill bit can drill before becoming dull, and to enhance the rate of penetration of such drill bits. Such improvements translate directly into reduction of drilling expense.